The present invention relates to an aqueous two-component polyurethane coating composition comprising
I.) a component (I) which contains as binder (A)
(A1) at least one water-soluble or water-dispersible polyester resin (A1) which contains hydroxyl groups and acid groups which can be converted into the corresponding acid anion groups and has an OH number of from 30 to 250 mg of KOH/g and an acid number of from 5 to 150 mg of KOH/g, and PA1 (A2) at least one water-soluble or water-dispersible polyurethane resin (A2) which contains hydroxyl groups and acid groups which can be converted into the corresponding acid anion groups and has an OH number of from 20 to 200 mg of KOH/g and an acid number of from 5 to 150 mg of KOH/g, and PA1 (A3) at least one water-soluble or water-dispersible acrylate copolymer which contains hydroxyl groups and acid groups which can be converted into the corresponding acid anion groups, and/or an acrylated polyester and/or an acrylated polyurethane, which has an OH number of from 40 to 200 mg of KOH/g and an acid number of from 5 to 150 mg of KOH/g, and (A4) if desired, at least one further polymer (A4) and
II.) a component (II) which contains a polyisocyanate component (F1) as crosslinking agent.
The present invention also relates to a process for preparing these aqueous coating compositions and to their use as a clearcoat or as a topcoat and to their use for coating plastics.
For ecological and economic reasons, the paint industry is seeking to replace the amount of organic solvents used in paints as far as possible by water. Aqueous coating compositions are already in use not only in the area of automotive line coating but also in the area of automotive repair finishes. In the area of plastic coating, too, it is increasingly desirable to use aqueous systems, not only in the area of primers but also in the area of topcoats.
Topcoats are understood to mean paints which are used for producing the uppermost paint coat. The uppermost paint coat can be a single-coat or multicoat system, in particular a two-coat system. Two-coat topcoats consist of a pigmented basecoat layer and a clearcoat layer applied on top of the basecoat layer, which clearcoat layer is unpigmented or pigmented only with transparent pigments. Nowadays two-coat paints are produced by the wet-in-wet method in which a pigmented basecoat is precoated and the resulting basecoat layer, without being subjected to a baking step, is overcoated with a clearcoat, and basecoat layer and clearcoat layer are then jointly cured. This method is very advantageous in terms of economics, but it makes high demands on the basecoat and the clearcoat. The clearcoat applied on top of the not yet cured basecoat must not dissolve the basecoat layer on the surface or interfere in any other way since this would lead to paints having poor appearance. This is in particular true of paints in which basecoats containing effect pigments (for example metallic pigments, in particular aluminum flakes, or nacreous pigments) are used.
In the area of plastic coating an additional requirement is that the resulting coatings should have high flexibility while exhibiting high moisture resistance (for example low permeability). Furthermore, the coatings should have good appearance. This means that the coatings exhibit, for example, high gloss and that the coating compositions show good flow properties. Furthermore the coatings should possess good adhesion. In addition, component (I) of the coating composition should have a long shelf life.
In the area of plastic coating, an additional requirement is that the coating compositions used can be cured at low temperatures (in general below 100.degree. C.) and give films having the desired properties even when cured at these low temperatures.
DE-A-4,421,823 discloses aqueous polyurethane coating compositions consisting of at least three components where the component (I) contains at least one binder dissolved in organic solvent, which binder is selected from the group consisting of polyester resins, polyurethane resins, polyacrylate resins and, if desired, of further binders; component (II) contains at least one uncapped polyisocyanate as crosslinking agent; and component (III) is essentially free of binder and contains water. The coating compositions are prepared by mixing the three components a short time before applying the coating compositions. These coating compositions are used, in particular, in the area of automotive repair finishes.
However, these coating compositions disclosed in DE-A-4,421,823 have the disadvantage that their preparation is quite expensive since three different components have to be stored and mixed with one another a short time before applying the coating compositions. Furthermore, this coating composition contains a binder component which has been predissolved in an organic solvent, this solvent being also present in the coating composition prepared from the three components.
Furthermore, DE-A-4,326,670 discloses aqueous two-component polyurethane coating compositions based on an aqueous dispersion of at least one binder having a number-average molecular weight of 1000 to 100,000 and containing groups which are reactive towards isocyanate groups, calculated on the basis of an OH number of 20 to 250 and an acid number of 10 to 100, the acid functions of which are at least in part neutralized, and of a polyisocyanate as crosslinking agent. Examples of suitable binders include polyacrylate resins, polyester resins, polyurethane resins or (meth)acrylated polyester resins or (meth)acrylated polyurethane resins.
DE-A-4,326,670 does not describe the use of a mixture of at least one polyester and at least one polyurethane resin and of at least one polyacrylate resin or an acrylated polyester and/or an acrylated polyurethane as binder. The coating compositions disclosed in DE-A-4,326,670 have the disadvantage that their shelf life is not sufficiently long.
Accordingly, the object of the present invention is to provide aqueous two-component polyurethane coating compositions which are suitable for coating plastic substrates, in particular in the area of the production of topcoats. The resulting coatings should have, in particular, high elasticity while exhibiting high moisture resistance (for example low permeability). Furthermore, the coating compositions should meet the requirements usually demanded of coating compositions which are used for coating plastic substrates. Accordingly, the coating compositions should, for example, also give coatings having good appearance (good gloss, good flow properties, and the like) and good adhesion. Furthermore, component (I) of the coating composition should have a long shelf life.
Finally, the coating compositions should be curable at low temperatures (in general below 100.degree. C.) and should give films having the desired properties even when cured at these low temperatures.
Surprisingly, this object is achieved by means of the coating compositions of the type mentioned at the beginning, wherein the mixing ratio of the polyester resin (A1) to the polyurethane resin (A2) is between 95 parts by weight of polyester resin: 5 parts by weight of polyurethane resin and 5 parts by weight of polyester resin: 95 parts by weight of polyurethane resin.
The present invention also provides a process for preparing these coating compositions and relates to the use of the coating compositions as topcoat or clearcoat and to their use for coating plastics.
It is surprising and was not foreseeable that the use of a mixture of at least one polyacrylate resin and/or acrylated polyester and/or acrylated polyurethane resin and at least one polyester resin and at least one polyurethane resin as binder would result in aqueous two-component polyurethane coating compositions which give coatings which, not only with respect to flexibility but also simultaneously with respect to moisture resistance, have improved properties compared with coatings prepared by using at least one polyacrylate resin and/or acrylated polyester and/or acrylated polyurethane resin and either at least one polyester resin or at least one polyurethane resin.
An additional advantage is that the coating compositions according to the invention result in coatings having good appearance and good adhesion. Furthermore, component (I) of the coating composition exhibits a long shelf life, and the coating compositions show good application performance. Finally, the coating compositions are curable at low temperatures (in general below 100.degree. C.) and lead to films having the desired properties even when cured at these low temperatures.